If a hidden card is played to a battlefield and all units on that battlefield are killed by a spell, does the hidden card stay?

Ruling: The hidden card would be removed during cleanup if it is still face down when all units are killed. However, since all hidden cards gain the reaction keyword, you can respond to the spell by playing the hidden card (as long as it wasn't played the same turn).

Sequence:

• Opponent casts spell to kill units
• You can respond by playing the hidden card (if it wasn't played this turn)
• If you don't respond and the card stays hidden, it's removed during cleanup

Nuances:

• During a showdown, your battlefield can have 0 units and 1 hidden card temporarily
• During a showdown, if you regain focus & priority and both players pass, the hidden card is put in the trash during cleanup
• You cannot use the reaction ability on the same turn the card was played hidden